RNA interference (RNAi) is a mechanism in molecular biology where the presence of certain fragments of double-stranded RNA (dsRNA) interferes with the expression of a particular gene, which shares a homologous sequence with the dsRNA. RNAi is a gene silencing process that requires active participation of cellular machinery. Although the specific mechanism is poorly understood, it is known that the ribonuclease enzyme Dicer binds to and cleaves short double-stranded RNA molecules (dsRNA) to produce double-stranded fragments of 21-23 base pairs with two-base single-stranded overhangs on each end. The short double-stranded fragments produced by Dicer, called small interfering RNAs (siRNAs), are then separated, presumably by an enzyme with helicase activity, and integrated into a multiprotein complex called the RNA-induced silencing complex (RISC).
Synthetic siRNAs and short hairpin RNAs (shRNAs) can be designed to have identical function. Whereas, siRNA are 2 strands of complementary RNA that can be synthesized, a shRNA is encoded by DNA as a single RNA molecule that hybridize to itself with a loop at one end. The loop is then cleaved intracellularly yielding a molecule similar to a siRNA. There are thousands of RNAi sequences available that are capable of downregulating gene expression. (See, e.g. Behlke, 2006, Mol Ther vol. 13 p 644). This method has become a universally accepted means of downregulating expression of any gene in mammalian cells.
Presently, RNAi molecules are delivered via electroporation, cationic- and liposome-mediated transfection, viral delivery, and direct injection (Behlke, 2006, Mol Ther vol. 13 p 644). One group has shown that bacteria can be used to deliver RNAi molecules to mammalian cells to screen for targeting siRNA molecules (Zhao et al., 2005, Nat Methods vol 2 p 967).
Antigen-presenting cells (APCs) like dendritic cells (DCs) are a major target for manipulation of immune responses and they have been modified using RNAi (Li et al., 2004, Immu Res vol 30 p 215). However, there is no available method that permits guaranteed co-delivery of multiple antigens and RNAi molecules to the same APC.